As implantable medical devices, in general, and cardiac pacers, in particular, have become more complex in operation, it has also become desirable to non-invasively transfer data both to and from the implantable device and an external device. The transfer of signals from the external device to the implanted device to modify the operating parameters thereof is referred to in the art as programming. Data is also transferred from the implanted device to the external device to provide various monitoring information. These transfers of signals and data to and from the implanted devices is referred to as telemetry.
In the field of programmable implanted medical devices, such as, for example, cardiac pacemakers, tachyarrhythmia control devices, implantable drug dispensers, nerve simulators, and the like, it has become common to provide an interactive transceiver system for both remotely programming operating functions, modes and parameters of the implanted device and receiving data from the device related thereto on command using radio-frequency telemetry systems. In nearly all such active electronic implanted devices, it has become highly desirable to have the ability to reprogram the device's modes of operation, parameters and other functions, and to monitor the performance thereof, both historically and contemporaneously. Such implanted electronic devices are designed to provide two-way telemetry by radio frequency signal transmission and reception between the implanted device and an antenna disposed in a programming head or wand of the external programming device to provide for the exchange of transmitted information, thereby enabling the aforementioned programming by telemetry-in and the reading out of data stored in the device by telemetry-out. The transmission of signals has generally been accomplished using a single coil antenna on the external device to transmit and receive signals to and from a single coil internal antenna of the implanted device.
Because the implanted device is usually powered by a primary cell, it is important to limit the energy consumption and current drain required for data transmission in order to conserve the cell's energy for use in its intended work function, i.e., in the case of a cardiac pacer, stimulating and sensing cardiac muscle. Thus, the current state of the art is to transfer signals between the implanted device and the external programmer using a radio frequency carrier employing very close spacing of the transmitting and receiving antennae. Such close spacing provides low-power operation for a given minimum signal-to-noise ratio in accordance with the well-known inverse square law.
The maximum required data transmission distance in such systems is on the order of two to three inches. This limitation of the required transmission distance allows containment of the transmission power to a few microwatts from the primary cell of the implanted device. Additionally, in the low-power data link described, the external receiver system must be of a very high gain. The high-gain requirement renders such systems vulnerable to noise generated by extraneous fields, generated by myriad interfering sources found in the modern electric- and magnetic-field environment. All forms of electronic systems, such as, for example, computers, printers, cathode ray viewing tubes and lighting control systems, are potential interfering sources to implant data transmission systems.
If an extraneous field has energy components in the pass band of the system employed to receive data from the weak field emanating from the implant, and if the extraneous field strength is in the range transmitted from the implant, it is very difficult to mitigate the interfering effect on the data link. Many prior art systems have attempted to overcome the adverse effects of noise created by extraneous fields by increasing the gain of the antennae of the telemetry system. This solution has many drawbacks, including increased power consumption. Furthermore, these prior systems do nothing to mitigate the effect of the extraneous fields, they merely attempt to overpower the noise.